From Sketch to Plug: The First Physical Step in Making a Fibreglass Chair

What a plug actually is

A plug — also called a master pattern — is a full-size positive model of the part you intend to make. It is not the chair. It is not the mould. It is the object the mould is taken from, which means it is the last chance to change the shape with a sanding block instead of a saw.

In composites tooling the logic runs in one direction: the plug defines the mould, the mould defines the part. Surface finish, dimensional fidelity and long-term repeatability all trace back to the plug, which is why pattern shops treat it as the real product of the early phase rather than scaffolding on the way to one.

Practically, the plug for a lounge chair is often a shaped block — foam, MDF, plywood ribs with skinned infill, sometimes a printed model — brought to a mirror finish and sealed so a mould can be laid over it without sticking, sagging or picking up texture.

• Plug: positive master pattern, built once, usually consumed or stored
• Mould (tool): negative taken off the plug, used repeatedly
• Part: positive again, taken from the mould, finished for the customer

Not every fibre product starts this way

Why the process you choose decides whether a plug exists at all

It's worth being precise, because "fibreglass" covers very different production routes. Hand lay-up and spray-up both use an open mould and therefore need a pattern upstream. Vacuum infusion also uses a mould surface, but adds a vacuum bag and resin feed, which changes the tool's requirements more than its origin. Resin transfer moulding (RTM) uses two matched tools, so the pattern work is doubled and tolerances tighten. Pultrusion uses a heated steel die to pull continuous profiles — no plug, no mould in the sculptural sense, and no freedom of form beyond a constant cross-section.

That last distinction matters for anyone specifying furniture. A pultruded bench leg and a hand-laid chaise shell are both "fibreglass," but only one of them began as a shape somebody carved and sanded.

KOLAYFIBER works in hand lay-up with gelcoat, which is why the plug stage is where our time concentrates. It is a labor-intensive route with a specific advantage: single-off and small-batch sculptural forms are economically possible, because the tooling is built by hand rather than machined from steel. The trade-off is honest — hand lay-up rewards patience at the pattern stage and punishes shortcuts there more than almost any other method.

• Hand lay-up — open mould, plug required, best for low volumes and sculptural form
• Spray-up — open mould, chopped fibre, faster, less control over laminate orientation
• Vacuum infusion — closed bag over a mould, higher fibre fraction, more process discipline
• RTM — matched tools, two surfaces, tighter tolerances, higher tooling cost
• Pultrusion — constant-profile sections through a heated die, no pattern stage

Reading the sketch for things a plug can't do

Before anything is cut, the drawing gets interrogated for release. Can the mould come off this shape in one piece? If the seat curls back under itself at the front edge, that's an undercut, and the tool now needs to split — which means a flange, a parting line, and a seam that someone will have to fair out on every part forever.

Draft angle is the next question. Vertical walls technically release, but they fight. The common workshop convention is to tilt anything near-vertical by a degree or two away from the pull direction, and to let the designer choose where that tilt hides. On a chair, it usually hides in the outer flank of the seat shell where the eye reads curvature anyway. That's a convention, not a law — plenty of parts release from zero draft with enough wax and patience.

Radii get the same treatment. A crisp internal corner is easy to draw and miserable to laminate: fabric bridges, resin pools, and the finished part shows a dark, brittle line. Softening that corner on the plug is cheaper than arguing with it on fifty parts. The curves people admire in finished pieces like the Wave Chaise are partly aesthetic and partly the geometry of what a hand can actually consolidate.

• Mark the pull direction before choosing the split line, not after
• Note every undercut in the drawing, even decorative ones
• Agree on the minimum internal radius early — it's a joint design and production decision
• Decide where a visible seam is acceptable; on a seat back, it rarely is

Substrate: what the plug is made of

Polyurethane or EPS foam blocks are the usual starting point for organic shapes: fast to carve, light to move, and forgiving of revision. Foam alone won't take a gloss, so it gets skinned — typically with a laminate layer and a filler/primer system that can be sanded hard without breaking through.

Sheet goods — MDF, plywood, or ribs with battens and bondo — suit plugs with developable surfaces and straight runs: counters, benches, plinths. Machined or printed patterns are increasingly common for smaller pieces, and they bring their own problem, layer lines, which have to be filled rather than polished away.

The rule across all of them is the same: whatever is underneath must be stable and must not move when the mould is laid over it and cures. Resin generates heat. Foam that wasn't properly sealed will reveal that fact at the worst moment.

• Carved foam — organic, sculptural forms; always needs a skin
• MDF / plywood — flat and ruled surfaces; seal the edges or they drink filler
• Printed patterns — accurate geometry, visible layer lines, needs filling
• Clay over armature — good for exploration, poor for anything that must survive a week

Tolerances: the error stack nobody mentions in the brief

Dimensional error accumulates through the chain. The plug carries whatever deviation the carving left. The mould adds the thickness of release layers and any movement during cure. The part adds its own cure shrinkage and the simple fact that a demoulded shell relaxes slightly when it comes off the tool. Thermoset polyesters shrink as they cure; the amount varies by resin system and laminate, which is exactly why pattern makers check critical dimensions against a part, not against the CAD.

The useful habit is to decide, early, which dimensions are critical and which are free. On a chair: seat height at the front edge, the footprint where it meets the floor, and any interface with a cushion or a bolt-down detail are critical. The exact sweep of the backrest is free — nobody measures it, and the eye forgives a millimeter.

Flatness of the base is the dimension most often neglected and most often complained about. A shell that rocks on a terrace floor reads as a defect regardless of how good the surface is. That's checked on the plug, with a straightedge, before any mould exists.

• Critical: contact footprint, seat height, mounting points, mating edges
• Free: overall sweep, decorative curvature, surface transitions
• Check on the first part too — the plug alone doesn't prove the chain

Surface prep: the ladder and the seal

Every flaw on the plug becomes a flaw in the mould, and every flaw in the mould becomes a flaw in every part. So the plug is sealed with a coating heavy enough to hide the filler beneath it and hard enough to polish — tooling gelcoat and high-build primers are both used, and the top coat is deliberately thin, so the sanding underneath has to be genuinely flat before it goes on.

A typical sanding progression works coarse to fine: roughly 80–120 grit for shaping, 220–320 for smoothing, 400–600 and up for finishing, then wet sanding to 1000 grit or beyond, and finally buffing and polishing to gloss. Skipping a step doesn't save time; it just moves the scratches to a stage where they're harder to find and harder to remove.

Light is the real inspection tool. Raking a single lamp across the surface at a low angle shows waviness that overhead light hides completely. Pattern makers tend to trust a palm over a shape more than their eyes, too — the hand picks up a flat spot in a curve long before the camera does.

Once the gloss is there, release comes last: wax, often several coats buffed between applications, sometimes a PVA film over it. Then the mould is laid up on top, and the plug has done its job.

• 80–120 grit: shaping
• 220–320: smoothing
• 400–600+: finishing
• Wet sand to 1000+, then buff and polish
• Raking light at every stage, not only at the end

What this means if you're specifying rather than building

If you are an architect or designer commissioning a piece, the plug stage is where your influence is highest and cheapest. Changing a seat height before the pattern is skinned costs an afternoon. Changing it after the mould is finished means a new mould.

It also explains why bespoke sculptural fibreglass prices the way it does: the first piece carries the pattern and tooling, and the second one doesn't. That's a structural fact of open-mould work, not a pricing policy. It's why repeat forms — a bench run for a hotel terrace, a planter family across a courtyard — behave very differently on a budget than a single one-off.

Seeing it helps. Our atelier page shows the shop where the plug, mould and lay-up stages sit in one building, and past projects show what the sequence produces at full scale. If the finish question is what's holding you up, the sample box answers it faster than a render will. For a sense of how pattern geometry shows up in a finished object, compare the long uninterrupted sweep of the Sweep Bench with the closed, inflated volume of the Kavun Sofa — two very different plug problems, two very different split-line decisions.

In short

A fibreglass chair doesn't begin with fibreglass. It begins with a solid object that will never be sold, carved and filled and sanded through a ladder of grits until it holds light cleanly, with draft where the mould has to release and generous radii where fabric has to lie down. Everything downstream — the mould, the gelcoat, the part a client eventually sits on — is a copy of that first object. Which is why, in hand lay-up work, the plug is not a preliminary step. It's the step.

If you're developing a seating form and aren't sure where it should split or how much draft it needs, send the sketch — we'll tell you honestly what the pattern stage would involve before anyone commits to it.